1. Field of the Invention
The present invention relates to a method of fabricating doped conjugated polyelectrolytes and their implementation in organic electronic or optoelectronic devices.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers/numerals within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers/numerals can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Conjugated polyelectrolytes (CPEs) are defined by a backbone that contains a π-conjugated electronic structure with pendant ionic functionalities [1-2]. They are fascinating objects of study, as they combine the optical and charge transport properties of organic semiconductors with the possibility of modulating physical properties via electrostatic interactions. Water-soluble CPEs have found utility as optical reporters in biosensors and bio-imaging applications [3-8]. Furthermore, due to their solubility in polar solvents, it is possible to fabricate multilayer optoelectronic devices in combination with neutral conjugated polymers. CPE interlayers have thus proven useful in organic solar cells [9-12], organic light-emitting diodes [13-14], and organic thin film transistors (OTFTs) [15-16].
Cationic narrow bandgap conjugated polyelectrolytes with a backbone containing alternating 4,4-bis-alkyl-4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene and 2,1,3-benzothiadiazole structural units, see FIG. 1(a), were recently reported [17]. For the specific case of PCPDTBT-Pyr+BIm4−, one observes unexpected n-type transport in film field-effect transistors. It was proposed that the pendant cationic functionalities lower both the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), thus stabilizing the radical anions in the conjugated backbones.
Bulk heterojunction (BHJ) solar cells comprising conjugated polymer donor and fullerene acceptor offer promising advantages such as low cost, light weight, and flexibility [43-45]. In addition to new materials design [46-49] and morphology optimization [50-51], interface engineering on BHJ solar cells is fundamentally important to enhance the power conversion efficiency (PCE) and device stability [52-53]. To improve the charge selectivity at the electrodes and minimize the energy barrier for charge extraction, a hole-transporting layer (HTL) with electron-blocking properties is inserted between the anode and BHJ active layer, and an electron-transporting layer (ETL) with hole-blocking properties is inserted between the cathode and BHJ active layer. The highly doped polymer Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS) is the most commonly used HTL material for organic BHJ solar cells, because of its solution processability, work function, sufficient conductivity, and high optical transparency in the visible-near infra-red (NIR) regime [52]. However, the strong anisotropy in the electrical conduction in spin-coated PEDOT:PSS layers, originating from their lamellar structures, often limits the charge collection in solar cells [54-57]. In addition, the acidic and hygroscopic nature of PEDOT:PSS tends to induce chemical instability between the active layer and electrodes [58-59]. To overcome these deficiencies, several types of materials have been explored to serve as HTLs in BHJ solar cells, including conducting polymers [60], metal oxides [49, 61-62], conjugated polyelectrolytes (CPEs) [63], cross-linkable materials [64], and graphene-based materials [65].
CPEs used as an interfacial material for organic photovoltaics have received increasing attention with the proven ability of improving the PCE through solution processing [63, 66-70]. In the past, conjugated polyelectrolyte (CPE) layers have been utilized as ETLs to improve electron extraction toward the cathode [66-70]. The advantages of CPEs as ETLs in BHJ solar cells include reduction of the series resistance, increase of the internal built-in voltage, and modification of the electron extraction properties. As a result, the short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF) can be selectively [66, 69, 70], or even simultaneously enhanced on a single device [67]. More recently, CPE interlayers have been also applied on the bottom cathode in inverted cells, leading to a record PCE approaching 10% based on BHJ solar cells using a single layer structure [70]. In contrast, the function of CPEs in affecting the hole injection/extraction has been rarely addressed [63]. One reason is that direct depositing CPE on top of the ITO substrate can often result in a decrease in the electrode's work function due to interfacial dipole interactions [71], resulting in hole extraction barrier. Another limitation is that most of the CPEs show relatively low electrical conductivity.